The present invention relates to a method for protecting the reheater surface of a circulating fluidized bed combustion system during an upset condition in which fluid flow to the reheater is interrupted.
Fluidized bed combustion has gained favor for a number of reasons. An outstanding feature is its ability to burn high-sulfur fuels in an environmentally acceptable manner without the use of flue-gas scrubbers. In fluidized-bed combustion, much of the sulfur contained in the fuel is removed during combustion by a sorbent material in the fluid bed, usually limestone. In this process, the production of nitrogen oxides is low because of the low temperature at which the combustion reaction takes place.
One type of fluidized bed combustion is the circulating fluidized bed system. In this system, the gas velocities in the furnace are three to four times as high as in conventional bubbling fluidized bed system. The small solid particles are carried up through the furnace and a uniform lower-density gas/solids mixture exists throughout the entire furnace. Since the solids move through the furnace at much lower velocities than gas, significant solids residence times are obtained. The long residence time coupled with the small particle size produce high combustion efficiency and high sulfur oxide removal with lower sorbent limestone feed.
In the circulating fluidized bed combustion system, the solids which are carried from the furnace are separated from the gas by a cyclone. The solids discharged from the bottom of the cyclone pass through a seal pot or syphon seal. In some designs, a portion of the solids can be directed to a fluid bed heat exchanger with the remainder being reinjected directly back into the furnace. The heat extracted from the solids in the fluid bed heat exchanger may be used to provide additional evaporation, superheat and/or reheat.
In order to prevent excessive moisture from forming in the low pressure steam turbine stages, it is conventional to interrupt the expansion process, remove the steam for reheating at constant pressure, and return it to the low pressure turbine stages. This is known as a reheat cycle. In a circulating fluidized bed system, this reheat may be preformed in the convection pass of the furnace, in the fluid bed heat exchanger or a combination of these. When the heat recovery fluid bed system is used for reheat, either alone or in combination with reheat in the convection pass, a problem exists when there is an upset condition, such as the loss of power or turbine trip, where fluid flow to the reheater is interrupted but where the reheater surface continues to be exposed to a heat source.